This invention relates to printed circuit boards, and especially to printed circuit boards which facilitate the removal and replacement of components.
It is common to mount complex electronic components on printed circuit boards by inserting pins of the components into plated through-holes of the circuit and soldering them in place. The plated through-holes normally provide connections between the pins and conductive material forming part of the circuit and situated at different levels of the board.
If it is necessary to remove a component, for example to replace it because it is faulty, it is found that the heat used to melt the solder tends to cause the board to expand and damage the circuitry of the board. The problem is especially serious with modern very-large-scale integrated-circuit components mounted in so-called pin grid array packages. The large number of densely packed pins in such an array (typically above 100) requires the solder for all the pins to be melted simultaneously if the process is not to be inconveniently long. The great amount of heat that is therefore applied, and the relatively great thickness of the board which is needed to provide the large number of interconnections required, then exacerbate the expansion problem and hence the damage to the board. Yet, because those components are expensive, it is all the more desirable to repair a board rather than to discard it. It has indeed been suggested that the only solution to the problem is to use sockets, which are soldered into the board. The components are then plugged into the sockets, but this method increases both the cost and bulk of the board and reduces its reliability and electrical performance.